In the same way as for NOx, CO, and also HC etc., restrictions on the volume of Particulate Matter (hereinafter referred to as “PM”) discharged from diesel engines grow stricter every year. Simple engine improvements in response to such intensification of regulations are not sufficient to realize full compliance. Techniques for accumulating the PM discharged from engines in a filter known as a Diesel Particulate Filter (hereinafter referred to as “DPF”) and for reducing the volume thereof through being externally discharged are also being developed.
Ceramic monolithic-honeycomb wall-flow type filters, mesh-type filters using ceramic or metallic fibers, and the like are available as such DPFs for direct accumulation of PM. In the same way as other exhaust gas purifying systems, an exhaust gas purifying system using a DPF is disposed at an intermediate position along the engine's exhaust passage, and purifies exhaust gases generated in the engine.
The exhaust pressure acting on the DPF increases in proportion to the volume of PM accumulation therein. Accordingly, any PM accumulation in the DPF must be periodically removed through a process such as combustion, thus regenerating the DPF. A large number of methods have been proposed for use in the regeneration process—for example, electrical-heater types, burner types, and reverse cleaning types.
However, when any of these regeneration methods are utilized, energy must be externally provided in order to facilitate PM combustion. And accordingly, problems such as deteriorated fuel efficiency, difficulty in maintaining control during regeneration, and the need for a double DPF system performing alternate PM accumulation and PM combustion (or DPF regeneration) are experienced. This in turn leads to the problem of the need for increasingly large and complex systems.
In order that these problems may be resolved, a DPF regeneration technique that performs PM oxidation utilizing heat of exhaust gases discharged by an engine has been proposed. In this technique, the PM oxidation temperature is reduced through the use of an oxidation catalyst, and accordingly, there is no need for external energy to be input. As the DPF regeneration process can continuously regenerate DPF at a fundamental level, a DPF system utilizing this technology is referred to as a Continuous Regeneration DPF System. Such a system is advantageous in that it features a single simplified DPF system, and also simplifies the control required in that DPF regeneration.
FIG. 8 presents an NO2 regeneration DPF system 1X as an example. In the NO2 regeneration DPF system 1X, the PM is oxidized with nitrogen dioxide (NO2), thus realizing regeneration. In this system, an oxidation catalyst 3Aa that oxidizes the nitrogen oxide (NO) in the exhaust gas is disposed on the upstream side of the standard wall flow filter 3Ab. Accordingly, almost all of the NOx in the exhaust gas, having passed through the oxidation catalyst 3Aa, has become NO2. The PM accumulation in the wall flow filter 3Ab disposed on the downstream side is oxidized by this NO2, becoming carbon dioxide (CO2). In this way, the PM is subsequently removed. Since this NO2 has a lower energy barrier than O2, the PM oxidation temperature (and the DPF regeneration temperature) can be reduced. Accordingly, continuous combustion of PM can be carried out using thermal energy contained in the exhaust gas without the need for any externally provided energy.
Note that in FIG. 8, E is a diesel engine, 2 is an exhaust passage, 4 is a fuel pump system, 5 is an electronic control system, 7 is a battery, 8 is a silencer, and 9 is a fuel tank.
Furthermore, FIG. 9 presents more improved NO2 regeneration DPF system 1Y, than that of FIG. 8. In this improved system 1Y, a porous catalytic coat layer 31 of oxidation catalyst 32A is applied to the porous wall 30 of wall flow filter 3B. As a result of this configuration, oxidation of NO and oxidation of the PM by the NO2 produced in that reaction both occur on the wall surface of the wall flow filter 3B. Accordingly, in this configuration the system has been even more simplified.
FIG. 10 shows an additional system 1Z. In this system 1Z, a porous catalytic coat layer 31 of an oxidation catalyst 32A and a PM oxidation catalyst 32B of oxide and the like is applied to the porous wall 30 of the wall flow filter 3C. As a result of this configuration, the PM accumulation in the filter 3C is combusted at a low temperature, and continuous regeneration of DPF is carried out.
As these DPF systems with catalysts utilize the oxidation reaction of PM by catalysts and NO2, the start temperature of PM oxidation is lower than that of normal filters. Accordingly, continuous regeneration of DPF is facilitated.
Nevertheless, even though the start temperature of PM oxidation is lowered, exhaust temperature of approximately 350° C. is required. For this reason, since the exhaust temperature is low during low-load driving, idling, and the like, PM oxidation and DPF self-regeneration do not take place. Consequently, in case that the engine remains in a state of idling, low-load driving, or the like, conditions for initiating PM oxidation are not achieved, even if PM accumulates. Therefore, exhaust pressure rises, and fuel efficiency is deteriorated as a result. Furthermore, there is a danger of engine stopping and other similar problems.
In these continuous regeneration DPF systems, conditions signaling the requirement for DPF regeneration have been setup. Determination of whether or not these conditions have been satisfied can be carried out by calculating the volume of PM accumulation in the filter based on engine's driving conditions, and/or by estimating the volume of PM accumulation based on corresponding filter pressure loss. Furthermore, when these conditions have been satisfied, control necessary for DPF regeneration is carried out. As a result of this control, the temperature of the exhaust gas is raised in a forcible manner, forced combustion of the accumulated PM is carried out, and the PM is removed.
In order to perform forced combustion of PM in a continuous regeneration DPF system, an electronic-control fuel injection system such as the common-rail is used to perform the following type of DPF regeneration control.
First of all, delay multi-step injection is carried out. Specifically, this comprises multi-step injection of small injection quantity prior to main injection and significant delay of main injection. By performing injection in this way, the exhaust temperature is raised above the activation temperature of the oxidation catalyst.
After raising the exhaust temperature through delay multi-step injection, normal injection control is restored, and through post injection, exhaust-pipe injection, or the like, fuel such as diesel oil (HC) is added to the inside of the exhaust pipe. This fuel is combusted by using an oxidation catalyst disposed on the upstream side. As a result of this combustion, the temperature of the exhaust gas flowing into the filter disposed on the downstream side is raised above the temperature required for forced combustion of the accumulated PM. Accordingly, the PM accumulation in the DPF is forcibly combusted and removed.
Furthermore, another method has been proposed. In this method, exhaust throttling is also carried out in addition to rise the temperature of the exhaust gas through multi-step injection. This dual process raises the engine's exhaust pressure, increases the amount of residual exhaust gas in the intake process, and in addition, raises the temperature of the exhaust gas. As a result of this increase in temperature of the exhaust gas, the ignition performance and the combustion performance of the injection fuel are improved. Accordingly, the exhaust temperature is increased.
An exhaust gas purifying system for a diesel engine that uses this method has been proposed in Japanese patent application Kokai publication No. 1992-81513. In this system, an exhaust throttle valve is provided on the downstream side of the trap filter (DPF). Furthermore, the degree of opening of the exhaust throttle valve is controlled while the filter is performing regeneration in order to maintain the exhaust temperature within a given regeneration temperature range, or in other words, in order to adjust the exhaust temperature at the inlet of the DPF to a given temperature.
However, in the control of DPF regeneration characteristic of the prior art, in which multi-step injection is used in combination with this exhaust throttling, when controlled driving is carried out for the purpose of regeneration, there is a problem of excessively large torque fluctuation. Furthermore, this control is also problematic in that white smoke is emitted.
In other words, during controlled driving for this regeneration, normal injection control is restored after the exhaust temperature has been raised beyond the activation temperature of the oxidation catalyst by multi-step injection performed in combination with exhaust throttling. Accordingly, when normal injection control is restored, torque fluctuation occurs as a result of sudden changes in exhaust pressure and large changes in injection timing. Furthermore, since the temperature of the exhaust gas entering the oxidation catalyst lowers by performing the normal injection, there are cases in which the multi-step injection in combination with exhaust throttling needs to be restored again from the normal injection control. However, torque fluctuation occurs in such cases as well.
Furthermore, when switching from multi-step injection in combination with exhaust throttling to normal injection control, the corresponding changes in injection quantity can lead to the generation of HC and white smoke.